As one of the processes of producing fine metal particles having an extremely small particle size, e.g., fine metal particles having an average particle size of 100 nm or less, Japanese Patent Application Laid-Open No. 3-34211 discloses a dispersion in which fine metal particles of 10 nm or less prepared by a gas evaporation method are dispersed in a dispersion solvent in a colloidal state and a process for producing the same. Japanese Patent Application Laid-Open No. 11-319538 and other references disclose a dispersion in which fine metal particles having an average particle size of several nm to several 10 nm prepared by a wet process making use of a reduction/deposition method using an amine compound for the reduction are dispersed in a colloidal state and a process for producing the same. The surface of the fine metal particles (metal nanoparticles) having an average particle size of several nm to several 10 nm prepared by a wet process disclosed in Japanese Patent Application Laid-Open No. 11-319538 and other references is covered with a polymer resin and the like to maintain the colloidal state.
It is generally known that fine metal particles having an average particle size of several nm to several 10 nm are sintered at a temperature well below the melting point (for example, even at 200° C. or lower in the case of silver fine particles (nanoparticles) having a clean surface). This is because, when the particle size of fine metal particles (nanoparticles) is sufficiently small, the proportion of atoms in a high energy state present on the particle surface increases in the whole particle, and the surface diffusion of metal atoms increases to a level that cannot be ignored, and as a result, the boundary of grains grows due to this surface diffusion to cause sintering.
When the surfaces of fine metal particles having an average particle size of several nm to several 10 nm are directly contacted, the fine metal particles are fused with each other and form an aggregate, which deteriorates uniform dispersibility in a dispersion solvent. For this reason, the surface of fine metal particles is uniformly covered with alkylamine or the like so that the particles have a surface covering molecular layer, and thus the fine metal particles exhibit high dispersibility.
On the other hand, in recent electronics-related fields, miniaturization of wiring patterns on wiring boards to be used is underway. In addition, regarding metal thin film layers used for forming various electrode patterns, utilization of metal thin film layers having an extremely thin film thickness has been developed. For example, upon forming a fine wiring or a thin film by screen printing, a dispersion of fine metal particles having an extremely small particle size is applied for drawing of ultrafine patterns or formation of thin film coating layers having an extremely thin film thickness. At present, fine particle dispersions of gold and silver applicable to the above purposes are already on the market.
More specifically, regarding processes for forming an ultrafine wiring pattern using a fine metal particle dispersion, methodology has already been established, for example, for using gold fine particles or silver fine particles. For instance, by drawing an extremely fine circuit pattern using a dispersion for ultrafine printing containing gold fine particles or silver fine particles and by subsequent sintering of the fine metal particles, wiring having a line width and a line space of 5 to 50 μm and a volume specific resistivity of 1×10−5 Ω·cm or lower can be formed on the obtained sintered product wiring layer.
Further, as the line width and the line space are reduced, disconnection due to electromigration has become another problem. In particular, in the part with a small wiring film thickness and a narrow line width formed at a step having differences, the wiring film thickness at the step edge tends to be thinner than that in other parts, and thus the current density is locally increased and frequency of occurrence of disconnection due to electromigration is increased. To avoid disconnection caused by this electromigration phenomenon, use of copper wiring is effective. For example, with much higher integration, utilization of copper materials for wiring patterns on semiconductor devices is being extensively studied.